Numerous industries and many applications utilize reciprocating pumps for transporting fluids. For example, reciprocating pumps are found in industries such as shipping, processing, manufacturing, irrigation, gasoline supply, air conditioning systems, flood control, marine services, etc. Conventional reciprocating pumps may employ one or more piston systems comprised of a plurality of pistons and associated piston chambers in driving the pump. Conventionally, as pistons displace longitudinally within the piston chamber, one or more volumes between the piston and the piston chamber increase or decrease, depending on the direction of longitudinal displacement. The increasing volumes must be filled with a fluid, such as air or other fluid, when the volume increases, and the fluid must subsequently be exhausted from the volume when the volume is decreased.
Conventional pumps have provided one or more vent lines, which may also be characterized as an exhaust port, which couple a volume of the piston chamber with the atmosphere surrounding the pump. One example of such a configuration is illustrated in FIGS. 1 and 2 in U.S. Pat. No. 7,458,309. As a shift piston displaces in a direction reducing the volume coupled to the vent line, a central body portion of the shift piston having substantially the same diameter as the interior of the piston chamber forces air through the vent line and into the surrounding atmosphere. Likewise, as the shift piston displaces in a direction increasing the volume coupled to the vent line, the central body portion of the shift piston pulls air into the piston chamber from the surrounding atmosphere.
Such piston systems are generally adequate for use in certain relatively benign environments. However, in some very abrasive environments, abrasive materials may enter into the exhaust ports and into the volume of the piston chamber, causing the piston and the piston chamber to wear at an increased rate. In other environments, chemicals or other materials in the surrounding atmosphere may enter into the exhaust ports and subsequently interfere with the motion of the pistons by causing the pistons to bind with and stick to the walls of their associated piston chamber or even to seize within the piston chamber.